Mapping coral

Knowing where past coral reefs existed is a crucial component in understanding the health of existing reefs, and hypothesising which ones will still exist in the future

Centuries-old nautical charts, mapped by sailors to avoid shipwrecks, can be a valuable asset in the ongoing research into the decline of our planet’s coral reefs. Specifically, understanding the health of coral reefs of the Florida Keys has been greatly helped by comparing 18th century imperial nautical maps – such as the above by George Gauld, a surveyor with the British Admiralty during the period 1773 to 1775 – with modern-day observations of coral reefs, allowing for far broader and longer term analysis of areas of the ocean where reefs were not known to have previously existed.

Researchers from a joint US-Australian study recently concluded a study utilising the data provided by these maps, and discovered that more than half of the coral reef habitat mapped in the 1770s no longer exists, while in some areas, coral loss was close to 90 per cent. ‘Most of the change that we found was in the near-shore,’ reveals Professor Loren McClenachan, Assistant Professor at Colby College, in Waterville, Maine, and lead author of the study. ‘Most of the coral that is still there from the 1770s was offshore, so there it’s still coral. But the near-shore reef is – to a large extent – lost.’

These results indicate that many reefs will have been demolished by direct human activities such as coastal development and dredging, unlike the climate change-induced ocean warming which is proving so destructive today. Hence, says McClenachan, these results should be considered an addition to the current dramatic coral losses.

McClenachan’s initial intentions were to try and map changes which have occurred to the entire Caribbean Sea since the 1700s. While this Florida Keys research has been a focusing of those ambitions, it does still give vital clues regarding the state of the rest of the sea, and how much it is similarly likely to have deteriorated over the past two-and-a-half centuries.

‘Interestingly, it’s the earlier maps that are better at providing ecological information,’ continues McClenachan, delving into the motivations of Gauld and his fellow colonial surveyors. ‘What they were really after is depth, and these were the first attempt at getting that information. One of the things we thought about trying to do was trace it through time, and you can’t, because the coral only really shows up on the 18th century maps. By the 1820s, there’s a lot more depth information, but they stopped including natural history information.’

This was published in the November 2017 edition of Geographical magazine.